Recent advances in terephthalic acid (TPA) manufacturing processes require relatively high p-xylene purity (99.7+%) in order to improve the quality of the product and reduce the costs of manufacturing. This is because such processes use hydrogenation as the main method for purifying the crude terephthalic acid produced in the oxidation section of the processes. Although the hydrogenation method is very selective for elimination of the major impurity, 4-carboxybenzaldehyde (4-CBA), by converting it to p-toluic acid, such methods can tolerate only very small amounts of 4-CBA (preferably less than 3,000 ppm).
The small amount of 4-CBA (or 3-CBA in isophthalic acid (IPA) production) can not be oxidized to TPA (or IPA) in the oxidizer, because the solvent used in the oxidizer, acetic acid, is a poor solvent to significantly dissolve TPA (or IPA) and 4-CBA (or 3-CBA). In fact, in traditional processes almost all the TPA (or IPA) and 4-CBA (or 3-CBA) produced are precipitated in the oxidizer to form a slurry. Therefore, a small amount of 4-CBA (or 3-CBA) is encapsulated inside of the TPA (or IPA) solids, and can not be further oxidized by air to form TPA (or IPA) in the oxidizer. It should be noted that even though the 4-CBA (or 3-CBA) is an aldehyde which can easily be oxidized to TPA (or IPA) by air under the oxidizer operating conditions, since it is present as a solid, it is not readily oxidized. Thus, there remains a need for a method for reduction of 4-CBA or 3-CBA from crude TPA or crude IPA, respectively, without encountering the disadvantages outlined above.